Valence Engineering via Selective Atomic Substitution on Tetrahedral

Subscriber access provided by IDAHO STATE UNIV

Article

Valence Engineering via Selective Atomic Substitution on Tetrahedral Sites in Spinel Oxide for Highly Enhanced Oxygen Evolution Catalysis Yan Liu, Yiran Ying, Linfeng Fei, Yi Liu, Qingzhao Hu, Guoge Zhang, Sin Yi Pang, Wei Lu, Chee Leung Mak, Xin Luo, Limin Zhou, Mingdeng Wei, and Haitao Huang J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b13701 • Publication Date (Web): 22 Apr 2019 Downloaded from http://pubs.acs.org on April 22, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 11 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Journal of the American Chemical Society

Valence Engineering via Selective Atomic Substitution on Tetrahedral Sites in Spinel Oxide for Highly Enhanced Oxygen Evolution Catalysis Yan Liu†‡, Yiran Ying†‡, Linfeng Fei†‡, Yi Liu₴, Qingzhao Hu†, Guoge Zhang£, Sin Yi Pang†, Wei Lu¥, Chee Leung Mak†, Xin Luo†∥*, Limin Zhou∇*, Mingdeng Wei∆, Haitao Huang†* †Department

of Applied Physics, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China. Prof. Y. Liu ₴National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, P.R. China. Prof. G. Zhang £School of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, P.R. China. Dr. W. Lu ¥University Research Facility in Materials Characterization and Device Fabrication, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China. Prof. L. Zhou ∇Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China Prof. M. Wei ∆Institute of Advanced Energy Materials, Fuzhou University, Fuzhou 350002, P.R. China. ABSTRACT: A major challenge which prohibits the practical application of single/double-transition metal (3d-M) oxides as oxygen evolution reaction (OER) catalysts is the high overpotentials during the electrochemical process. Herein, our theoretical calculation shows that Fe will be more energetically favorable in the tetrahedral site than Ni and Co, which can further regulate their electronic structure of binary NiCo spinel oxides for optimal adsorption energies of OER intermediates and improved electronic conductivity, and hence boost their OER performance. X-ray absorption spectroscopy study on the as-synthesized NiCoFe oxide catalysts indicates that Fe preferentially dopes into tetrahedral sites of the lattice, which induces high proportions of Ni3+ and Co2+ on the octahedral sites (the active sites in OER). Consequently, this material exhibits a significantly enhanced OER performance with an ultralow overpotential of 201 mV cm-2 at 10 mA cm-2 and a small Tafel slope of 39 mV dec-1, which are much superior to state-of-the-art Ni-Co based catalysts.

INTRODUCTION Water splitting (H2O → H2 + 1/2O2) by electrolysis is a sustainable and portable strategy to store electrical energy in chemical bonds. However, it is a thermodynamically unfavorable process, whose overall efficiency is limited by the sluggish four proton-electron transfer kinetics in oxygen evolution reaction (OER).1 Noble metal oxides, such as RuO2 and IrO2, are state-of-the-art electrocatalysts to surmount the high overpotential (η), but the scarcity and costliness hamper their massive applications. Recently, much attention has been redirected to low-cost and earthabundant first-row transition metals (TMs, such as nickel (Ni), cobalt (Co), iron (Fe), zinc (Zn), or manganese (Mn)) in forms of metal oxides/(oxy)hydroxides,2 sulfides,3 phosphates,4 nitrides,5 carbides,6 and perovskites7 as next generation electrocatalysts. Studies have demonstrated that, compared with common unary-metal

oxides/(oxy)hydroxides, binary 3d-M oxide/(oxy)hydroxides (particularly those based on Ni, Co or Fe) show better OER activities due to their higher electrical conductivity and more cations with unsaturated coordination, which act as surface active centers for adsorption and activation of reaction intermediates.2b, 8 Smith et al. confirmed by in situ electrical measurements that Co1-xFex(OOH) is conductive (0.7–4.0 mS cm-1 at an overpotential of ~ 300 mV), but FeOOH is insulating (2.2×10-2 mS cm-1) and has a high overpotential beyond 400 mV.8g Due to the increase of exposed active sites, NiFe hydroxide catalyst2b, 8c, 8d is generally more active (η < 300 mV) for OER than either Ni or Fe alone8e, 8f, which shows overpotentials of > 400 and > 500 mV, respectively, at 10 mA cm-2. In spite of the above efforts on searching of suitable binary electrocatalysts, the performance of such catalysts is still far from satisfactory for practical

ACS Paragon Plus Environment

Journal of the American Chemical Society 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

applications, as their overpotentials during OER are significantly higher than those of noble metal catalysts. The formation of ternary metal oxides has been proposed as an effective strategy to regulate the electronic structure of binary 3d-M oxides/(oxy)hydroxides for enhanced OER activity.9 For instance, Sargent’s group recently demonstrated that non-3d high-valence metal (W) can modulate 3d-CoFe oxyhydroxides by migrating protons towards oxygen at Co sites, providing near-optimal energetics for OER and the as-prepared CoFeW oxyhydroxides exhibited an ultra-low overpotential (191 mV).9a Following this report, Sun9b and Jin et al.9c utilized the multivalent and high-valent properties of chromium (Cr, from +1 to +6) and vanadium (V, from +2 to +5) to synthesize NiFeCr and NiFeV double layered hydroxides (LDHs), respectively, and these trimetallic LDHs showed very excellent OER activity and were significantly more reactive than bimetallic NiFe LDHs. While most previous reports focused on metal (oxy)hydroxides (see examples above) due to simple synthesis protocols, the corresponding 3d-M oxides are of equal importance and worthy of careful study as catalysts, in view of their intrinsic structure stability and relatively high conductivity.10 It should be noted that ternary metal oxides are, in principle, much more complex than ternary metal (oxy)hydroxides (octahedral occupation), as their electronic structure and the OER performance are remarkably influenced by the exact site occupation (tetrahedral or octahedral site in spinel lattice) of the dopant cation11, which actually provides another degree of freedom in tuning the electronic structure of oxides for OER. Herein, we demonstrate in this work that the successful site-selective incorporation of Fe into binary NiCo spinel-type oxide leads to an excellent OER catalyst. Fe is particularly chosen due to the following reasons. (ⅰ) It has similar ionic radius to those of Ni or Co cation, which facilitates its substitution into the Ni and Co sites. (ⅱ) It has different number of 3d electrons to tune the electronic structure. (ⅲ) Fe3+ has a 3d5 electron configuration to fit in either octahedral or tetrahedral site, whereas Ni and Co cations have a 3d6~8 electron configuration favoring octahedral coordination (according to the crystal field stability energy)12 which is confirmed by our theoretical calculations. Therefore, Fe dopant would preferably enter into the tetrahedral site and adjust electronic structure of NiCo spinel oxide, as shown in Scheme 1. An outstanding OER activity in the Feincorporated NiCo2O4 (denoted as NiCoFeO) is predicted by our theoretical calculation of their electronic structures. Experimentally, we design an electrochemical codeposition approach to incorporate Fe cations into the lattice of NiCo2O4 (denoted as NiCoO) spinel with preferred tetrahedral occupation. The experimental results confirm the theoretical prediction, showing good electrocatalytic activity of NiCoFeO with an ultralow overpotential of only 201 mV cm-2 at 10 mA cm-2 and outstanding stability. This work opens a new pathway to engineer the electronic structure in multimetal oxides for efficient electrocatalysts.

Page 2 of 11

Scheme 1. Empirical site preference energies for some divalent and trivalent ions in the spinel structure.12 RESULTS AND DISCUSSION Firstly, we use theoretical calculation to illustrate the hidden mechanisms behind the OER process in the NiCoFeO, and compare it to that of unary NiO, Co3O4, FeOOH, binary NiCoO, NiFe2O4 (denoted as NiFeO), and Fe-doped Co3O4 (denoted as CoFeO), which are reported OER catalysts with good performance13. Density functional theory (DFT)-optimized crystal structures of those materials are listed in Figure S1 in Supporting Information. As shown in Figure 1a, DFT-calculated total energy of one Fe atom doping into the tetrahedral site of NiCoO spinel oxide unit cell is 0.25 eV lower than that doping into the octahedral site, confirming the preference of tetrahedral occupancy of incorporated Fe. The DFT-calculated H2O adsorption energies on the plane of (001) are shown in Figures 1b, where higher H2O molecule adsorptive ability in the ternary NiCoFeO is expected, which is favorable for enhancing the reactivity of active sites on the surface and OER activity13e. The optimized Gibbs free energy changes of intermediates during OER process on the surface of NiCoO and NiCoFeO are shown in Figure 1c. Step II is the potential determining step (PDS) for NiCoO and NiCoFeO where the value of the free energy change for NiCoFeO (2.20 eV) is smaller than that for NiCoO (2.41 eV). Besides, similar tendencies appear on the other low-index facets of (100), (110), and (111) for NiCoO and NiCoFeO (Figure S2). In other words, on these exposed facets, NiCoFeO will form a lower energy barrier of intermediates than NiCoO to drive water oxidation, indicating the significant role of Fe in enhancing electrocatalytic performance. Contour plot of overpotential as a function of Gibbs free energy changes in Figure S3 in Supporting Information clearly manifests that thermodynamics of OER process can be tailored by introducing new elements into the system. The ternary NiCoFeO exhibits the lowest theoretical overpotential among all the examined OER catalysts, and thus is predicted to have the most outstanding catalytic activity. Subsequently, we scrutinize the electronic structure of NiCoO and NiCoFeO systems to elucidate the essential physics behind the optimized thermodynamic behavior of NiCoFeO. It is well accepted that the near-unity eg electron occupancy14 can provide the optimal OER performance. This eg descriptor bases on the fact that TM-3d-eg orbitals have a strong overlap with oxygen 2p orbitals, forming σbonds which have a more significant influence on the electronic structure with the adsorbed oxygen during OER


Page 3 of 11 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


process than π-bonds which are formed by TM-3d-t2g and O2p orbital hybridization. Charge density difference distributions between catalyst with and without an adsorbed O atom for either NiCoO or NiCoFeO catalyst shows the shape of σ-bonds, confirming the fact that TM-3deg orbitals play a determinant role in the oxygen absorption on the catalysts (Figure S5, Supporting Information). To have more details in depth, we calculate the density of states (DOSs) in Figure 1d. Compared with NiCoO system, the total DOSs of NiCoFeO show larger carrier concentration and higher intrinsic electrical conductivity originating from more electronic states near the Fermi level, which are attributed to the strong TM-3d and O-2p orbital

hybridization after introducing Fe into the NiCoO system. Increased hybridization for NiCoFeO also reflects smaller energy difference between TM-3d and O-2p band centers than that of NiCoO (Table S1, Supporting Information), indicating boosted driving force of oxygen exchange between surface cations and O22–/O2– adsorbates during OER.15 Furthermore, we integrate the partial DOSs for TM3d-eg orbitals below Fermi level and find that the number of eg electrons per TM atom is 1.53 for NiCoFeO, which is closer to 1 than 1.62 for NiCoO. Therefore, the strategy of doping Fe into NiCoO provides a generalized approach to regulate the electronic structure and design new catalysts with better OER performance.

Figure 1. Theoretical investigation of the OER activity in transition metal oxides. a) Schematic illustration of preference of Fe doping into tetrahedral sites of NiCoO spinel oxides. b) Adsorption energy of H2O molecule onto the (001) surface of NiCoO, CoFeO and NiCoFeO spinels. c) Calculated Gibbs free energy profiles of OER on the (001) surface of NiCoO and NiCoFeO. The potential determining steps for the OER process on the surface of the catalysts are marked as PDS. d) Calculated partial density of states (PDOSs) of bulk NiCoO and NiCoFeO. PDOSs above zero represent spin-up states and those below zero represent spin-down states. The vertical dotted line represents Fermi energy level EF (set to zero).

Figure 2. Preparation and structure characterization of NiCoFe@NiCoFeO NTAs. a) SEM image. b) HRTEM image, where the yellow line shows the interface between NiCoFeO and NiCoFe. The inset: SAED pattern. c) HAADF image and corresponding EDX mapping



Page 4 of 11

of Ni, Co, Fe, and O. d) ICP-MS results for NiCo@NiCoO NTAs and NiCoFe@NiCoFeO NTAs through the comparison between actual ratio and feeding ratio of Ni:Co:Fe (Ni:Co=1:1 and Ni:Co:Fe=1:1:0.5, respectively). e) XRD patterns.

As illustrated in Scheme S1 in Supporting Information, we designed core-shell structured NiCoFe@NiCoFeO nanotube arrays on carbon fiber cloth (NiCoFe@NiCoFeO NTAs/CFC) via electrodeposition of ternary NiCoFe@NiCoFe layered double hydroxides (NiCoFe@NiCoFe LDHs, Figures S7and 8, Supporting Information) on a template of ZnO nanorod arrays (ZnO NRAs, Figure S6, Supporting Information) grown on CFC, followed by a thermal treatment. The as-fabricated NiCoFe@NiCoFeO NTAs (Figure 2a) have threedimensional structure with rough surface, which are similar to the morphology of NiCoFe@NiCoFe LDH NTAs/CFC. TEM images (Figures S9a and b, Supporting Information) show that the NiCoFe@NiCoFeO NTAs has a hollow tubular structure composed of nanosheets, whose surface is rougher than that of the NiCo@NiCoO NTAs (Figure S9c, Supporting Information), indicating increased active sites. The high-resolution TEM (HRTEM, Figure 2b) combined with selected area electron diffraction (SAED) analysis (the inset of Figure 2b) reveals the core/shell structure of NiCoFe@NiCoFeO NTAs, with a low crystallinity phase of NiCoFeO as the shell enveloping the Ni-Co-Fe metallic core. The high angle annular dark field (HAADF) image and corresponding energy dispersive X-ray spectroscopy (EDX) mapping images (Figure 2c) clearly demonstrate that Ni, Co, Fe, and O atoms are homogeneously distributed in the NTA matrix. Inductively coupled plasma mass spectrometry (ICP-MS, Figure 2d, Table S5, Supporting Information) shows that the molar ratio of Ni:Co:Fe in NiCoFe@NiCoFeO NTAs/CFC is about 1.17:1.00:0.45, close to the feeding ratio of 1.00:1.00:0.50. This suggests that the co-deposition of Ni, Co, and Fe can be realized in our synthesis process to prepare homogeneous ternary metal oxides with controlled composition. X-ray diffraction (XRD) patterns (Figure 2e, Figure S10a, Supporting Information) show the characteristic diffraction peaks of nickel, cobalt, and iron, which agree well with the observed XPS depth profiles of Ni 2p and Co 2p (Figures S11d to e, Supporting Information). The appearance of a broad peak at the diffraction angle of about 36.7o corresponds to the (311) plane of a cubic Fd3m inverse spinel NiCo2O4-like phase (JCPDS Card No. 200781).16 This is in line with our previous model in DFT calculation and XPS results (Figures S11a to c, Figure S12, Supporting Information). With the incorporation of iron, no additional diffraction peaks associated with the formation of other phases emerge, regardless of the variation in the Ni:Co:Fe ratio, indicating the atomic substitution of Ni and Co ions by Fe on the inverse spinel lattice sites. The lattice substitution by Fe cations in NiCo@NiCoO NTAs induced lattice distortion and isomorphous replacement.17 Additionally, compared with NiCo@NiCoO NTAs/CFC, the increase in diffraction peak width and the decrease in peak intensity reveal smaller crystallites and more structural disorders in NiCoFe@NiCoFeO NTAs/CFC.18 Similar results are also found in the corresponding SAED patterns (Figure S10b, Supporting Information). A single wide diffraction peak at around 11.3o (Figure S10c, Supporting Information) is observed, confirming the formation of LDH phase in the precursors of NiCo(Fe)@NiCo(Fe)O NTAs/CFC, which is also verified from the XPS results (Figure S13, Supporting

Information). Three expected Raman-active modes are observed: A1g (667 cm-1), Eg (479 cm-1) and F2g (188 cm-1) (Figure S10d, Supporting Information).19 Compared with NiCo@NiCoO NTAs/CFC, the A1g peak of NiCoFe@NiCoFeO NTAs/CFC is broadened due to increased degree of structural disorder induced by iron incorporation, which is consistent with observed XRD patterns. The intensity of the F2g peak was generally reduced due to the occupation of Fe3+ in the tetrahedral sites, which may affect the symmetry of the spinel structure.20 To explore the effect of Fe incorporation on the local atomic coordination and electronic structure of the NiCoFe@NiCoFeO NTAs, X-ray absorption fine structure (XAFS) spectra were recorded. The insets in Figure 3a and Figure 3b highlight the pre-edge feature of the Ni and Co Kedge X-ray absorption near-edge structure (XANES) spectra for the NiCo(Fe)@NiCo(Fe)O NTAs, where peaks A (~ 8,334.0 eV) and B (~ 7710.0 eV) are ascribed to the 1s → 3d orbital electronic transition (a dipole-forbidden but quadrupole-allowed transition) with 3d-4p orbital hybridization in tetrahedral NiO4 and CoO4, respectively.21 Their integrated pre-edge peak intensities for the NiCoFe@NiCoFeO NTAs are greatly weaker than that for NiCo@NiCoO NTAs, which are attributed to the substitution of tetrahedral Ni/Co by Fe in the inverse spinel structure of NiCo2O4.21-22 With reference to the Ni L-edge XANES for high-spin Ni2+ (octahedral) measured on NiO23 and low-spin Ni3+ (octahedral) taken from LaNiO323 (Figure S14a), the shift of the average peak in the NiCoFe@NiCoFeO NTAs to higher energy than that in the NiCo@NiCoO NTAs suggests increased average oxidation state of surface Ni atoms. Co Ledge XANES spectra demonstrate a smaller ratio of Co3+/Co2+ in octahedral sites in the NiCoFe@NiCoFeO NTAs (Figure S14b). These results essentially indicate that Ni and Co atoms are mainly located in octahedral sites with increased average valence of Ni and decreased average valence of Co in the surface of NiCoFe@NiCoFeO NTAs (i.e., the NiCoFeO shell), respectively. Fitted Fe L-edge XANES spectra corroborate the result that Fe is preferentially occupied in tetrahedral sites with 76.1 % Fe3+ and 14.2 % Fe2+ (Figure S15). In addition, O K-edge XANES spectra (Figure S16) feature a pre-edge character centered at ~530 eV that arises from the hybridization of Ni/Co-3d with O-2p orbitals. Increasing intensity of pre-edge peak for the NiCoFe@NiCoFeO can be explained by increased O-2p character in the metal 3d band which contributes to facilitating oxygen exchange between catalyst surface and intermediate species, and thereby fast OER kinetics.15 This result is well in agreement with our DFT calculation. The Ni and Co K-edge extended XAFS (EXAFS) oscillation curves in K-space for NiCoFe@NiCoFeO NTAs are different from those for NiCo@NiCoO NTAs, indicating different local atomic arrangement surrounding Ni and Co atoms (Figure S17).24 Compared with the NiCo@NiCoO NTAs, a significant oscillation reduction of Ni (Figures S18a and b) and Co (Figures S18c and d) K-edge Fourier transforms (FT) k3χ(k) EXAFS curves in R-space for NiCoFe@NiCoFeO NTAs are seen at higher coordination shells, which can be attributed to increased distortion of MO6 (M=Ni, Co).25 In addition, the


Page 5 of 11 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


NiCoFe@NiCoFeO NTAs have larger Debye-Waller factors as evaluated in the Ni and Co K-edge FT-EXAFS curves, which also reveals a higher degree of disorder, further

confirming the existence of the structural distortion both around Ni and Co atom centers (Tables S6 and 7). The increased

Figure 3. Electronic structure and chemical states of Ni and Co in NiCo@NiCoO NTAs and NiCoFe@NiCoFeO NTAs. a) Ni K-edge XANES spectra (Inset: zoomed-in view of the Ni K-edge XANES). b) Co K-edge XANES spectra (Inset: zoomed-in view of the Co Kedge XANES). Peaks A and C represent 1s → 3d transition. The grey shaded areas in Figure 3a and 3b show the intensity of the 1s →3d transition. XPS for c) Ni 2p and d) Co 2p. The shaded parts in Figure c) and d) show the peak convolution areas from Ni and Co elements with different valence states. Ni:Co:Fe=1:1:0.5.

coordination number of Ni-O and Co-O reflects the fact that, in the NiCoFe@NiCoFeO NTAs, more Ni/Co atoms occupy the octahedral sites, in agreement with the previous mentioned decrease of pre-edge peak intensity of Ni and Co K-edge XANES spectra. It is thus concluded that the Fe incorporation induces (ⅰ) an increase of Ni and a decrease of Co average valence states, respectively, and (ⅱ) the generation of nanocrystals with small and highly distorted crystal lattice. To further evaluate the change of oxidation states of metal cations in as-prepared samples, we investigated the difference in Ni and Co 2p XPS spectra between NiCo@NiCoO NTAs and NiCoFe@NiCoFeO NTAs. Figure 3c shows that the Ni 2p XPS peak of NiCoFe@NiCoFeO NTAs shifts significantly towards more positive value than that of NiCo@NiCoO NTAs, indicating increased atomic ratio of Ni3+/Ni2+ (from 0.31 to 0.62) on the surface. A shift of the Co 2p XPS peak of NiCoFe@NiCoFeO NTAs/CFC to higher energy is observed in Figure 3d, suggesting that the Co3+/Co2+ ratio in the NiCo@NiCoO NTAs/CFC surface is decreased. The atomic ratio of Co3+/Co2+ (1.15) for NiCoFe@NiCoFeO NTAs/CFC is obviously lower than that (1.3) for NiCo@NiCoO NTAs/CFC, suggesting that more Co2+ exists near the surface of NiCoFe@NiCoFeO NTAs/CFC. These results confirm previous XAFS results. From the

results of XAFS and XPS, it is reasonable to conclude that Fe incorporation into NiCo oxides is responsible for increased ratio of Ni3+/Ni2+ and decreased ratio of Co3+/Co2+ in the octahedral occupation (as active sites)14a, 26, respectively. On the basis of Yang’s principles14c and Xu’ reports14a, 27 developed from the former, Ni2+ (t2g6eg2), Co2+ (t2g5eg2), and Fe3+ (t2g3eg2) with high-spin state28 have the same eg occupancy, while Ni3+ (t2g6eg1) and Co3+ (t2g6eg0)29 belong to low-spin state.30 Since the content of Fe in active octahedral sites on the catalyst surface is below 0.2% (the surface of NiCoFeO contains only 2% of Fe ions (from XPS data), among which 9.7% of them are in the octahedral sites), the Fe contribution on eg occupancy can thus be neglected. Hence, increased ratio of Ni3+/Ni2+ will bring an eg occupancy of Ni closer to unity, which is high-activity configuration for OER31; meanwhile, since Co3+ percentages are much higher than Co2+ (from Co L-edge XANES data), decreased ratio of Co3+/Co2+ will bring an eg occupancy of Co approaching unity, which is the optimal eg filling for OER32. The predominance of 3d-M ions with an electronic configuration of eg orbital closer-to-unity occupancy on the NiCoFe@NiCoFeO NTAs/CFC surface contributes favorably to the highly efficient OER process due to moderate metaloxygen bond strength between metal cations and OER intermediates.



Page 6 of 11

Figure 4. Electrocatalytic OER measurement for NiCo@NiCoO NTAs/CFC and NiCoFe@NiCoFeO NTAs/CFC with different ratios of Ni:Co:Fe. a) LSV polarization curves and b) the corresponding Tafel plots. c) Tafel slopes and overpotentials of NiCoFe@NiCoFeO NTAs/CFC (Ni:Co:Fe=1:1:0.5) compared with those of the state-of-the-art OER catalysts, highlighted in the shaded region (Table S8). d) Chronopotentiometric (CP) response curves at a constant current of 10 mA cm-2.

The hollow structure and lamellate wall endow the nanotube arrays with improved performance in electrocatalytic reactions. Self-supported NiCoFe@NiCoFeO NTAs/CFC can be utilized as working electrodes, enabling the convenient evaluation of electrochemical activity towards OER without the need of extra substrate or additional binders. In the magnified cyclic voltammetry curve (Figure S19), the NiCoFe@NiCoFeO NTAs/CFC (Ni:Co:Fe=1:1:0.5) present clear redox current peaks prior to water oxidation, which is attributed to the conversion of Ni(Ⅱ)/Co(Ⅱ or Ⅲ) to higher oxidation state Ni(Ⅲ)/Co(Ⅲ or Ⅳ) species, respectively.2a, 33 In the linear sweep voltammetry (LSV) polarization curves (Figure 4a), the NiCoFe@NiCoFeO NTAs/CFC, regardless of various ratios of Ni:Co:Fe, possess a much higher OER activity than that of the NiCo@NiCoO NTAs/CFC. To generate a polarization current of 10 mA cm2, NiCoFe@NiCoFeO NTAs/CFC (Ni:Co:Fe=1:1:0.5) request an applied potential of 1.43 V (η=201 mV), while NiCo@NiCoO NTAs/CFC require a higher value of 1.72 V (η=490 mV). This result agrees with the DFT calculation, and further underpins the reduction of the thermodynamic overpotential of OER based on the principle of closer-tounity eg occupancy. Meanwhile, the NiCoFe@NiCoFeO NTAs/CFC (Ni:Co:Fe=1:1:0.5) also exhibit a more negative onset potential of 1.39 V than that of the NiCo@NiCoO NTAs/CFC (1.65 V). Furthermore, the NiCoFe@NiCoFeO NTAs/CFC (Ni:Co:Fe=1:1:0.5) show a much higher current density (80.2 mA cm-2) than that of the NiCo@NiCoO

NTAs/CFC at an applied potential of 1.53 V. The incorporation of Fe into NiCo@NiCoO NTAs/CFC improves the electrocatalytic activity. Figure 4b compares the corresponding Tafel plots to evaluate the reaction kinetics of the electrocatalysts. The measured Tafel slope for NiCoFe@NiCoFeO NTAs/CFC (Ni:Co:Fe=1:1:0.5) is 39 mV dec-1, much lower than that of NiCo@NiCoO NTAs/CFC (177 mV dec-1), confirming much faster electrochemical kinetics. The influence of Fe content on Tafel plots is also explored in Figure 4b, from which the NiCoFe@NiCoFeO NTAs/CFC (Ni:Co:Fe=1:1:0.5) demonstrate the best OER activity among all the electrocatalysts with various Ni:Co:Fe ratios. A smaller Tafel slope with decreased overpotential can provide more increased OER rate, beneficial for practical applications.33a In addition, double-layer capacitance (Cdl) measurements are performed to evaluate the electrochemically active surface areas (ECSAs). NiCoFe@NiCoFeO NTAs/CFC (Ni:Co:Fe=1:1:0.5) display a Cdl of 42.0 mF cm-2, higher than that of NiCo@NiCoO NTAs/CFC (20.3 mF cm-2), demonstrating larger exposed active surface area of the NiCoFe@NiCoFeO NTAs/CFC (Ni:Co:Fe=1:1:0.5) than that of the NiCo@NiCoO NTAs/CFC (Figure S20). Besides, the Brunauer–Emmett–Teller (BET) surface area is determined by nitrogen adsorptiondesorption isotherm test (Figure S22), where the BET surface area of NiCoFe@NiCoFeO NTAs/CFC (129 m2 g-1), is slightly higher than that of NiCo@NiCoO NTAs/CFC (118 m2 g-1). However, the amount of increase in either ECSA or BET surface area is not significant, indicating that Fe-induced


Page 7 of 11 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


modulation of electronic structure plays a dominant role in the improvement of OER activity for the NiCoFe@NiCoFeO NTAs/CFC, as shown in the normalized OER LSV curves by ECSAs (Figure S21) and by BET surface areas (Figure S23). Low overpotential and low Tafel slopes of the NiCoFe@NiCoFeO NTAs/CFC (Ni:Co:Fe=1:1:0.5) are comparable or even superior (Figure 4c) to most of the state-of-the-art non-precious metal based electrocatalysts (Table S8). Importantly, a lower charge transfer resistance (Rct, 4.4 Ω) of the NiCoFe@NiCoFeO NTAs/CFC is achieved than that of the NiCo@NiCoO NTAs/CFC (Figure S24). Especially, the NiCoFe@NiCoFeO NTAs/CFC (Ni:Co:Fe=1:1:0.5) exhibit a very stable OER activity with almost 100% potential retention (~ 0.1% increase) for OER with no significant structural changes (SEM, Figure S25) by continuous measurement of the potential for 12 hours (Figure 4d). NiCoFe@NiCoFeO NTAs/CFC generally show better stability as OER electrocatalysts than NiCo@NiCoO NTAs/CFC. It is reported that the structural distortion could improve the structural stability by reducing the surface energy.34 The increased structure distortion on the surface of the NiCoFe@NiCoFeO NTAs/CFC could introduce more catalytically active sites to enhance their electrocatalytic performance for OER. Table S9 and Figure S26 compare the electrocatalytic OER activities of NiCo@NiCoO NTAs/CFC and NiCoFe@NiCoFeO NTAs/CFC with different Ni:Co:Fe ratios. The results as a whole reveal that doping of Fe into NiCo@NiCoO NTAs/CFC improves the electrocatalytic activity through site-selective modulation of eg electrons, among which the NiCoFe@NiCoFeO NTAs/CFC (Ni:Co:Fe=1:1:0.5) exhibit the best catalytic performance for OER. We believe that our strategy may also be applicable to the nonaqueous oxygen evolution reaction,35 which can be a feasible approach to the development of highly active OER catalysts for Li-O2 batteries, Na-O2 batteries, K-O2 batteries, and so on. CONCLUSION In conclusion, we have developed a facile Ni-Co-Fe codeposition method to prepare ternary NiCoFe@NiCoFeO NTAs/CFC as highly efficient catalysts for OER. More notably, the incorporation of Fe into the binary Ni-Co inverse spinel lattice can modulate the 3d electronic structure by preferential Fe-doping on the Ni/Co tetrahedral sites, as predicted by DFT calculation and further confirmed by XAFS characterization. Compared with NiCo@NiCoO NTAs/CFC, significantly enhanced electrocatalytic activity of NiCoFe@NiCoFeO NTAs/CFC for OER can be explained by a closer-to-unity eg occupancy, increased degree of structural disorder and increased electrical conductivity. The optimized ternary NiCoFe@NiCoFeO NTAs/CFC (Ni:Co:Fe=1:1:0.5) catalyst achieves an outstanding activity for OER with an ultralow overpotential of 201 mV cm-2 at 10 mA cm-2 and a small Tafel slope of 39 mV dec-1 in 1 mol L-1 KOH solution, demonstrating one of the most active Ni-Co based electrocatalysts for OER. This work manifests the importance of Fe doping in the binary Ni-Co spinel oxide on improving the intrinsic activity of electrocatalysts. This finding suggests the value of site-selective atomic doping in the development of multinary 3d-M oxides, where the electronic structure can be effectively modulated and the

OER catalytic activity can be improved. The strategy is promising for guiding rational design of low-cost, yet efficient multifunctional catalysts (for example, Mn-based NiCo2O4, Cr-based MnCo2O4, etc.) for board applications in rechargeable air-based devices for energy storage. EXPERIMENTAL SECTION Materials. Zn(NO3)2·6H2O, NH4NO3, CoSO4·7H2O, NiSO4·6H2O, FeCl3, ZnSO4, and KOH were all analytical grade purity from Aladdin Reagent (Shanghai) Co., Ltd. and were used as received without further purification. Preparation of ZnO NRAs on CFC. ZnO NRAs were grown on CFC by electrodeposition in the solution of 1 mmol L-1 Zn(NO3)2·6H2O and 5 mmol L-1 NH4NO3 at a current density of 0.8 mA cm-2 for 1.5 h at 75 C. During the electrodeposition process, the electrolytic cell was kept sealed to avoid water evaporation. After the deposition, CFC appeared white with the coating. As-deposited CFC was then washed thoroughly by deionized water and ethanol. Preparation of NiCoFe@NiCoFe LDH/ZnO NRAs on ZnO. NiCoFe@NiCoFe LDH/ZnO NRAs were fabricated by electrodeposition of NiCoFe LDHs on the ZnO NRA-coated CFC under the optimized conditions (obtained in electrochemical test; Figure S27), with a current density of 2 mA cm-2 for 1.5 h at 50 oC. The electrolyte was 60 mL of metal ion solution with various Ni2+:Co2+:Fe3+:Zn2+ feeding concentration ratios of 1:1:0.25:1, 1:1:0.5:1, 1:1:1:1 (Zn2+ forms ZnO that acts as a hard template, Figure S28). The deposited CFC was rinsed with deionized water and ethanol for several times. Finally, the obtained materials were dried at 60 C for 24 h. For comparison, NiCo@NiCo LDH/ZnO NRAs (Ni:Co:Zn=1:1:1) in the absence of any iron was prepared in the same way. Preparation of NiCoFe@NiCoFe LDH NTAs on CFC. The ZnO template was completely removed by immersing the above sample in 1 mol L-1 NaOH for 6 h to produce NiCoFe@NiCoFe LDH NTAs/CFC. For comparison, NiCo@NiCo NTAs/CFC (Ni:Co:Zn=1:1:1) were prepared by the same method. Fabrication of NiCoFe@NiCoFe NTAs on CFC. The asprepared NiCoFe@NiCoFe LDH NTAs/CFC were calcined in air at 300 ℃ for 2 h with a heating rate of 2 ℃ min-1 to give NiCoFe@NiCoFeO NTAs/CFC. For comparison, NiCo@NiCoO NTAs/CFC (Ni:Co:Zn=1:1:1) were prepared by the same method. Characterization. Scanning electron microscope (SEM, TM 3000, Hetachi, Japan) and transmission electron microscope (TEM, JOEL JEM-2010, Japan) were used for microstructure and morphology characterizations. X-ray diffraction (XRD, Rigaku D/max IIIA, Cu kα, λ=0.15418 nm, Japan) was performed for crystalline structure analysis. Inductively coupled plasma-mass spectrometry (ICP-MS, Agilent 7700x, USA) was used to determine the compositions of the products. Raman spectra were taken on a HORIBA HR800 Raman spectrometer at 488 nm. X-ray photoelectron spectroscopy (XPS, Thermo Microlab 350) was employed to study the surface composition of Ni-Co hybrid NTAs inside an ultrahigh vacuum system. Depth profile XPS was used to analyze the elemental distribution of the core-shell structure of NiCoFe@NiCoFeO NTAs deposited on a Ti foil, along the direction perpendicular to



the substrate. The sample was etched at a rate of 3 nm minby Ar+ ions. The X-ray absorption near-edge spectra (XANES, Ni and Co K-edge) were performed in fluorescence mode for the bulk at the 1W1B beamline station in Beijing Synchrotron Radiation Facility (BSRF, Beijing). The storage rings of BSRF was operated at 2.5 GeV with a maximum current of 250 mA. Ni, Co, and Fe L-edge XANES data based on the total electron yield (TEY) mode were measured at the BL12B beamline station with an electron bean energy of 800 MeV and a maximum current of 300 mA in National Synchrotron Radiation Laboratory of China (NSRL, Hefei). XANES studies via TEY mode reveal the valence states of the materials surface (1-5 nm deep). The normalization based on the post-edge was realized by the ATHENA module implemented in the IFEFFIT software packages. The acquired extended X-ray absorption fine structure spectra (EXAFS) data were also processed using the abovementioned module. Nitrogen adsorption/desorption isotherms were performed at 77 K (liquid nitrogen) to determine BET surface area with a Micromeritics ASAP2020 volumetric adsorption analyzer. Electrochemical Characterization. Cyclic voltammetry (CV) curves, linear sweep voltammograms (LSVs) and electrochemical impedance spectroscopy (EIS, 5 mV amplitude 0.01~10k Hz frequency) tests were recorded by Solartron 1470E (AMETEK, U.S.A.) with a classical threeelectrode configuration (platinum as counter electrode and saturated calomel as reference electrode) in 1 mol L-1 NaOH. The current density was normalized to the geometrical surface area, and the measured potentials vs. SCE were converted to a reversible hydrogen electrode (RHE) scale according to the Nernst equation (ERHE=ESCE + 0.2415 + 0.0591 × pH). To obtain steady-state OER polarization curves, CVs test was performed at a scan rate of 100 mV s-1 at least for 50 cycles. To eliminate the possible effect of the capacitive/oxidation currents on the LSV and Tafel plot measurements, a reverse scan (from high to low overpotential) of the polarization curve was conducted at a scan rate of 2 mV s-1 from 0.7 to 0 V. All polarization curves were corrected for the iR compensation within the cell. Cycling stability test was carried out at a constant current of 10 mA for 12 h (1×1 cm active CFC area). Calculations. Density Functional Theory (DFT) calculations were performed using Vienna ab initio simulation package (VASP)36 employing the projectoraugmented wave (PAW) method. The exchange-correlation function was calculated using the generalized gradient approximation (GGA) in the Perdew-Burke-Ernzerhof (PBE)37 implementation. Hubbard U framework and Dudarev rotationally invariant approach38 were applied to correct the DFT self-interaction errors for strongly correlated electrons in the first-row transition metal ions (in this work, Ni, Co, and Fe). The values of U-J (Ueff) were set as 6.4 eV for Ni, 4.5 eV for tetrahedral Co, 6.5 eV for octahedral Co, and 4.0 eV for Fe respectively based on previous work39. A plane wave expansion with a cut-off energy of 450 eV was used for all calculations. K-points sampling with a Monkhorst-Pack mesh40 scheme was set as 3×3×1 and 5×5×1 respectively for structural relaxation and self-consistent field calculation in adsorption energies, and 5×5×5 for calculation of density of states (DOSs) of bulk 1

Page 8 of 11

materials. Spin polarization was considered in all calculations. The atomic positions were allowed to relax until the energy were less than 1×10-5 eV and absolute value of the Hellmann-Feynman force less than 0.02 eV Å-1. A vacuum region with a thickness of 12 Å was constructed in the z direction to minimize the interaction between adjacent image cells. In structural relaxation, both the adsorbates and the top two layers of atoms were allowed to relax, with the rest of atoms frozen at their bulk position.

ASSOCIATED CONTENT Supporting Information ESAS estimation method, DFT calculation details, schematic illustration of material design, SEM and TEM characterizations, XRD pattern, Raman spectra, XPS depth profile, ICP-MS, Ni, Co, and Fe-L edge XANES spectra, O-K edge XANES spectra, Ni and Co-K edge EXAFS and FT-EXAFS, and details of electrochemical measurement. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Email: [email protected] (X.L.). *Email: [email protected] (L.Z.). *Email: [email protected] (H.H.).

Present Addresses ∥School

of Physics, Sun Yat-sen University, Guangzhou, Guangdong Province, P.R. China, 510275

Author Contributions ‡These

authors contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. PolyU152665/16E) and the Hong Kong Polytechnic University (Project Nos. Q54V, 1-ZVGH, and G-UABC).

REFERENCES (1) Jiao, Y.; Zheng, Y.; Jaroniec, M.; Qiao, S. Z., Design of electrocatalysts for oxygen-and hydrogen-involving energy conversion reactions. Chem. Soc. Rev. 2015, 44, 2060-2086. (2) (a) Zhu, Y. P.; Ma, T. Y.; Jaroniec, M.; Qiao, S. Z., Self-templating synthesis of hollow Co3O4 microtube arrays for highly efficient water electrolysis. Angew. Chem. Int. Ed. Engl. 2017, 56, 13241328. (b) Gong, M.; Li, Y.; Wang, H.; Liang, Y.; Wu, J. Z.; Zhou, J.; Wang, J.; Regier, T.; Wei, F.; Dai, H., An advanced Ni-Fe layered double hydroxide electrocatalyst for water oxidation. J. Am. Chem. Soc. 2013, 135, 8452-8455. (3) Hou, Y.; Qiu, M.; Nam, G.; Kim, M. G.; Zhang, T.; Liu, K.; Zhuang, X.; Cho, J.; Yuan, C.; Feng, X., Integrated hierarchical cobalt sulfide/nickel selenide hybrid nanosheets as an efficient threedimensional electrode for electrochemical and photoelectrochemical water splitting. Nano Lett. 2017, 17, 42024209. (4) Wang, P.; Song, F.; Amal, R.; Ng, Y. H.; Hu, X., Efficient water splitting catalyzed by cobalt phosphide-based nanoneedle arrays supported on carbon cloth. ChemSusChem 2016, 9, 472-477. (5) Jia, X. D.; Zhao, Y. F.; Chen, G. B.; Shang, L.; Shi, R.; Kang, X. F.; Waterhouse, G. I. N.; Wu, L. Z.; Tung, C. H.; Zhang, T. R., Ni3FeN nanoparticles derived from ultrathin NiFe-layered double


Page 9 of 11 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


hydroxide nanosheets: an efficient overall water splitting electrocatalyst. Adv. Energy Mater. 2016, 6, 1502585. (6) Tang, Y. J.; Liu, C. H.; Huang, W.; Wang, X. L.; Dong, L. Z.; Li, S. L.; Lan, Y. Q., Bimetallic carbides-based nanocomposite as superior electrocatalyst for oxygen evolution reaction. ACS Appl. Mater. Interfaces 2017, 9, 16977-16985. (7) Bockris, J. O.; Otagawa, T., Mechanism of oxygen evolution on perovskites. J. Phys. Chem. 1983, 87, 2960-2971. (8) (a) Bajdich, M.; Garcia-Mota, M.; Vojvodic, A.; Norskov, J. K.; Bell, A. T., Theoretical investigation of the activity of cobalt oxides for the electrochemical oxidation of water. J. Am. Chem. Soc. 2013, 135, 13521-13530. (b) Friebel, D.; Louie, M. W.; Bajdich, M.; Sanwald, K. E.; Cai, Y.; Wise, A. M.; Cheng, M. J.; Sokaras, D.; Weng, T. C.; Alonso-Mori, R.; Davis, R. C.; Bargar, J. R.; Norskov, J. K.; Nilsson, A.; Bell, A. T., Identification of highly active Fe sites in (Ni,Fe)OOH for electrocatalytic water splitting. J. Am. Chem. Soc. 2015, 137, 1305-1313. (c) Li, N.; Bediako, D. K.; Hadt, R. G.; Hayes, D.; Kempa, T. J.; von Cube, F.; Bell, D. C.; Chen, L. X.; Nocera, D. G., Influence of iron doping on tetravalent nickel content in catalytic oxygen evolving films. Proc. Natl. Acad. Sci. USA 2017, 114, 14861491. (d) Zhang, W.; Wu, Y. Z.; Qi, J.; Chen, M. X.; Cao, R., A thin NiFe hydroxide film formed by stepwise electrodeposition strategy with significantly improved catalytic water oxidation efficiency. Adv. Energy Mater. 2017, 7, 1602547. (e) Louie, M. W.; Bell, A. T., An investigation of thin-film Ni–Fe oxide catalysts for the electrochemical evolution of oxygen. J. Am. Chem. Soc. 2013, 135, 12329-12337. (f) Trotochaud, L.; Young, S. L.; Ranney, J. K.; Boettcher, S. W., Nickel–iron oxyhydroxide oxygen-evolution electrocatalysts: the role of intentional and incidental iron incorporation. J. Am. Chem. Soc. 2014, 136, 6744-6753. (g) Burke, M. S.; Kast, M. G.; Trotochaud, L.; Smith, A. M.; Boettcher, S. W., Cobalt–iron (oxy)hydroxide oxygen evolution electrocatalysts: the role of structure and composition on activity, stability, and mechanism. J. Am. Chem. Soc. 2015, 137, 3638-3648. (9) (a) Zhang, B.; Zheng, X.; Voznyy, O.; Comin, R.; Bajdich, M.; Garcia-Melchor, M.; Han, L.; Xu, J.; Liu, M.; Zheng, L.; Garcia de Arquer, F. P.; Dinh, C. T.; Fan, F.; Yuan, M.; Yassitepe, E.; Chen, N.; Regier, T.; Liu, P.; Li, Y.; De Luna, P.; Janmohamed, A.; Xin, H. L.; Yang, H.; Vojvodic, A.; Sargent, E. H., Homogeneously dispersed multimetal oxygen-evolving catalysts. Science 2016, 352, 333-337. (b) Li, P.; Duan, X.; Kuang, Y.; Li, Y.; Zhang, G.; Liu, W.; Sun, X., Tuning electronic structure of NiFe layered double hydroxides with vanadium doping toward high efficient electrocatalytic water oxidation. Adv. Energy Mater. 2018, 8, 1703341. (c) Yang, Y.; Dang, L.; Shearer, M. J.; Sheng, H.; Li, W.; Chen, J.; Xiao, P.; Zhang, Y.; Hamers, R. J.; Jin, S., Highly active trimetallic NiFeCr layered double hydroxide electrocatalysts for oxygen evolution reaction. Adv. Energy Mater. 2018, 8, 1703189. (10) (a) Li, F.; Liu, J.; Evans, D. G.; Duan, X., Stoichiometric synthesis of pure MFe2O4 (M = Mg, Co, and Ni) spinel ferrites from tailored layered double hydroxide (hydrotalcite-like) precursors. Chem. Mater. 2004, 16, 1597-1602. (b) Rives, V., Layered double hydroxides: present and future. Nova Science Publishers: 2001. (11) Szotek, Z.; Temmerman, W. M.; Kodderitzsch, D.; Svane, A.; Petit, L.; Winter, H., Electronic structures of normal and inverse spinel ferrites from first principles. Phys. Rev. B 2006, 74, 174431. (12) (a) Navrotsky, A.; Kleppa, O. J., The thermodynamics of cation distributions in simple spinels. J. Inorg. Nucl. Chem. 1967, 29, 2701-2714. (b) Burns, R. G.; Fyfe, W. S., Site of preference energy and selective uptake of transition-metal ions from a magma. Science 1964, 144, 1001-1003. (13) (a) Wang, J.; Li, K.; Zhong, H. X.; Xu, D.; Wang, Z. L.; Jiang, Z.; Wu, Z. J.; Zhang, X. B., Synergistic effect between metal-nitrogencarbon sheets and NiO nanoparticles for enhanced electrochemical water-oxidation performance. Angew. Chem. Int. Ed. Engl. 2015, 54, 10530-10534. (b) Grewe, T.; Deng, X.; Tüysüz, H., Influence of Fe doping on structure and water oxidation activity of nanocast Co3O4. Chem. Mater. 2014, 26, 3162-3168. (c) Chemelewski, W. D.; Lee, H. C.; Lin, J. F.; Bard, A. J.; Mullins, C. B., Amorphous FeOOH oxygen

evolution reaction catalyst for photoelectrochemical water splitting. J. Am. Chem. Soc. 2014, 136, 2843-2850. (d) Indra, A.; Menezes, P. W.; Sahraie, N. R.; Bergmann, A.; Das, C.; Tallarida, M.; Schmeisser, D.; Strasser, P.; Driess, M., Unification of catalytic water oxidation and oxygen reduction reactions: amorphous beat crystalline cobalt iron oxides. J. Am. Chem. Soc. 2014, 136, 1753017536. (e) Bao, J.; Zhang, X.; Fan, B.; Zhang, J.; Zhou, M.; Yang, W.; Hu, X.; Wang, H.; Pan, B.; Xie, Y., Ultrathin spinel-structured nanosheets rich in oxygen deficiencies for enhanced electrocatalytic water oxidation. Angew. Chem. Int. Ed. Engl. 2015, 54, 7399-7404. (14) (a) Wei, C.; Feng, Z.; Scherer, G. G.; Barber, J.; Shao-Horn, Y.; Xu, Z. J., Cations in octahedral sites: a descriptor for oxygen electrocatalysis on transition-metal spinels. Adv. Mater. 2017, 29, 1606800. (b) Hong, W. T.; Risch, M.; Stoerzinger, K. A.; Grimaud, A.; Suntivich, J.; Shao-Horn, Y., Toward the rational design of nonprecious transition metal oxides for oxygen electrocatalysis. Energy Environ. Sci. 2015, 8, 1404-1427. (c) Suntivich, J.; May, K. J.; Gasteiger, H. A.; Goodenough, J. B.; Shao-Horn, Y., A perovskite oxide optimized for oxygen evolution catalysis from molecular orbital principles. Science 2011, 334, 1383-1385. (15) Suntivich, J.; Hong, W. T.; Lee, Y.-L.; Rondinelli, J. M.; Yang, W.; Goodenough, J. B.; Dabrowski, B.; Freeland, J. W.; Shao-Horn, Y., Estimating hybridization of transition metal and oxygen states in perovskites from O K-edge X-ray absorption spectroscopy. J. Phys. Chem. C 2014, 118, 1856-1863. (16) Shen, L. F.; Che, Q.; Li, H. S.; Zhang, X. G., Mesoporous NiCo2O4 nanowire arrays grown on carbon textiles as binder-free flexible electrodes for energy storage. Adv. Funct. Mater. 2014, 24, 2630-2637. (17) Liu, X.; Chang, Z.; Luo, L.; Xu, T.; Lei, X.; Liu, J.; Sun, X., Hierarchical ZnxCo3–xO4 nanoarrays with high activity for electrocatalytic oxygen evolution. Chem. Mater. 2014, 26, 18891895. (18) Lu, Q.; Lattanzi, M. W.; Chen, Y.; Kou, X.; Li, W.; Fan, X.; Unruh, K. M.; Chen, J. G.; Xiao, J. Q., Supercapacitor electrodes with high-energy and power densities prepared from monolithic NiO/Ni nanocomposites. Angew. Chem. Int. Ed. 2011, 50, 68476850. (19) Zhong, J. H.; Wang, A. L.; Li, G. R.; Wang, J. W.; Ou, Y. N.; Tong, Y. X., Co3O4/Ni(OH)2 composite mesoporous nanosheet networks as a promising electrode for supercapacitor applications. J. Mater. Chem. 2012, 22, 5656-5665. (20) Yan, Z.; Keller David, A.; Rietwyk Kevin, J.; Barad, H. N.; Majhi, K.; Ginsburg, A.; Anderson Assaf, Y.; Zaban, A., Effect of spinel inversion on (CoxFe1−x)3O4 all ‐ oxide solar cell performance. Energy Technol. 2016, 4, 809-815. (21) Yamamoto, T., Assignment of pre-edge peaks in K-edge Xray absorption spectra of 3d transition metal compounds: electric dipole or quadrupole? X-Ray Spectrom. 2008, 37, 572-584. (22) Chadwick, A. V.; Savin, S. L. P.; Fiddy, S.; Alcantara, R.; Lisbona, D. F.; Lavela, P.; Ortiz, G. F.; Tirado, J. L., Formation and oxidation of nanosized metal particles by electrochemical reaction of Li and Na with NiCo2O4: X-ray absorption spectroscopic study. J. Phys. Chem. C 2007, 111, 4636-4642. (23) Bitla, Y.; Chin, Y. Y.; Lin, J. C.; Van, C. N.; Liu, R.; Zhu, Y.; Liu, H. J.; Zhan, Q.; Lin, H. J.; Chen, C. T.; Chu, Y. H.; He, Q., Origin of metallic behavior in NiCo2O4 ferrimagnet. Sci. Rep. 2015, 5, 1520115208. (24) (a) Zhao, Y.; Chen, G.; Bian, T.; Zhou, C.; Waterhouse, G. I.; Wu, L. Z.; Tung, C. H.; Smith, L. J.; O'Hare, D.; Zhang, T., Defect-rich ultrathin ZnAl-layered double hydroxide nanosheets for efficient photoreduction of CO2 to CO with water. Adv. Mater. 2015, 27, 7824-7831. (b) Wu, G.; Chen, W. X.; Zheng, X. S.; He, D. P.; Luo, Y. Q.; Wang, X. Q.; Yang, J.; Wu, Y.; Yan, W. S.; Zhuang, Z. B.; Hong, X.; Li, Y. D., Hierarchical Fe-doped NiOx nanotubes assembled from ultrathin nanosheets containing trivalent nickel for oxygen evolution reaction. Nano Energy 2017, 38, 167-174.



(25) Yin, S.; Tu, W.; Sheng, Y.; Du, Y.; Kraft, M.; Borgna, A.; Xu, R., A highly efficient oxygen evolution catalyst consisting of interconnected nickel-iron-layered double hydroxide and carbon nanodomains. Adv. Mater. 2018, 30, 1705106. (26) Jacobs, J. P.; Maltha, A.; Reintjes, J. G. H.; Drimal, J.; Ponec, V.; Brongersma, H. H., The surface of catalytically active spinels. J. Catal. 1994, 147, 294-300. (27) (a) Wang, T.; Sun, Y.; Zhou, Y.; Sun, S.; Hu, X.; Dai, Y.; Xi, S.; Du, Y.; Yang, Y.; Xu, Z. J., Identifying influential parameters of octahedrally coordinated cations in spinel ZnMnxCo2–xO4 oxides for the oxidation reaction. ACS Catalysis 2018, 8, 8568-8577. (b) Zhou, Y.; Sun, S.; Song, J.; Xi, S.; Chen, B.; Du, Y.; Fisher, A. C.; Cheng, F.; Wang, X.; Zhang, H.; Xu, Z. J., Enlarged Co-O covalency in octahedral sites leading to highly efficient spinel oxides for oxygen evolution reaction. Adv. Mater. 2018, 30, 1802912. (28) Rai, R. C.; Wilser, S.; Guminiak, M.; Cai, B.; Nakarmi, M. L., Optical and electronic properties of NiFe2O4 and CoFe2O4 thin films. Appl. Phys. A-Mater. Sci. & Proc. 2012, 106, 207-211. (29) Muhich, C. L.; Aston, V. J.; Trottier, R. M.; Weimer, A. W.; Musgrave, C. B., First-principles analysis of cation diffusion in mixed metal ferrite spinels. Chem. Mater. 2016, 28, 214-226. (30) (a) Appandairajan, N. K.; Gopalakrishnan, J., A study of Co3−xNixO4 (0≤x≤1) system. Proc. Indian Acad. Sci.-Sect. A, Chem. Sci. 1978, 87, 115-120. (b) Battle, P. D.; Cheetham, A. K.; Goodenough, J. B., A neutron diffraction study of the ferrimagnetic spinel NiCo2O4. Mater. Res. Bull. 1979, 14, 1013-1024. (31) Zhao, Y.; Jia, X.; Chen, G.; Shang, L.; Waterhouse, G. I.; Wu, L. Z.; Tung, C. H.; O'Hare, D.; Zhang, T., Ultrafine NiO nanosheets stabilized by TiO2 from monolayer NiTi-LDH precursors: an active water oxidation electrocatalyst. J. Am. Chem. Soc. 2016, 138, 65176524. (32) Liu, Y.; Cheng, H.; Lyu, M.; Fan, S.; Liu, Q.; Zhang, W.; Zhi, Y.; Wang, C.; Xiao, C.; Wei, S.; Ye, B.; Xie, Y., Low overpotential in vacancy-rich ultrathin CoSe2 nanosheets for water oxidation. J. Am. Chem. Soc. 2014, 136, 15670-15675. (33) (a) Zhuang, L.; Ge, L.; Yang, Y.; Li, M.; Jia, Y.; Yao, X.; Zhu, Z., Ultrathin iron-cobalt oxide nanosheets with abundant oxygen vacancies for the oxygen evolution reaction. Adv. Mater. 2017, 29, 1606793. (b) Kuang, M.; Han, P.; Wang, Q. H.; Li, J.; Zheng, G. F., CuCo hybrid oxides as bifunctional electrocatalyst for efficient water splitting. Adv. Funct. Mater. 2016, 26, 8555-8561. (c) Lu, Z.; Xu, W.; Zhu, W.; Yang, Q.; Lei, X.; Liu, J.; Li, Y.; Sun, X.; Duan, X., Threedimensional NiFe layered double hydroxide film for high-efficiency oxygen evolution reaction. Chem. Commun. (Camb.) 2014, 50, 6479-6482. (34) Liu, R.; Wang, Y.; Liu, D.; Zou, Y.; Wang, S., Water-plasmaenabled exfoliation of ultrathin layered double hydroxide nanosheets with multivacancies for water oxidation. Adv. Mater. 2017, 29, 1701546. (35) (a) Yao, K. P. C.; Lu, Y.-C.; Amanchukwu, C. V.; Kwabi, D. G.; Risch, M.; Zhou, J.; Grimaud, A.; Hammond, P. T.; Bardé, F.; ShaoHorn, Y., The influence of transition metal oxides on the kinetics of Li2O2 oxidation in Li–O2 batteries: high activity of chromium oxides. Phys. Chem. Chem. Phys. 2014, 16, 2297-2304. (b) Wang, Y.; Liang, Z.; Zou, Q.; Cong, G.; Lu, Y.-C., Mechanistic insights into catalyst-assisted nonaqueous oxygen evolution reaction in lithium–oxygen batteries. J. Phys. Chem. C 2016, 120, 6459-6466. (c) Lai, N.-C.; Cong, G.; Liang, Z.; Lu, Y.-C., A highly active oxygen evolution catalyst for lithium-oxygen batteries enabled by highsurface-energy facets. Joule 2018, 2, 1511-1521. (36) (a) Kresse, G.; Hafner, J., Ab initio molecular-dynamics simulation of the liquid-metal-amorphous-semiconductor transition in germanium. Phys. Rev. B: Condens. Matter 1994, 49, 14251-14269. (b) Kresse, G.; Furthmuller, J., Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Phys. Rev. B 1996, 54, 11169-11186. (37) Perdew, J. P.; Burke, K.; Ernzerhof, M., Generalized gradient approximation made simple. Phys. Rev. Lett. 1996, 77, 3865-3868.

Page 10 of 11

(38) (a) Anisimov, V. I.; Zaanen, J.; Andersen, O. K., Band theory and Mott insulators: Hubbard U instead of StonerI. Phys. Rev. B 1991, 44, 943-954. (b) Dudarev, S. L.; Botton, G. A.; Savrasov, S. Y.; Humphreys, C. J.; Sutton, A. P., Electron-energy-loss spectra and the structural stability of nickel oxide: an LSDA+U study. Phys. Rev. B 1998, 57, 1505-1509. (39) (a) O'Brien, C. J.; Rak, Z.; Brenner, D. W., Free energies of (Co, Fe, Ni, Zn)Fe2O4 spinels and oxides in water at high temperatures and pressure from density functional theory: results for stoichiometric NiO and NiFe2O4 surfaces. J. Phys.: Condens. Matter 2013, 25, 445008. (b) Zasada, F.; Gryboś, J.; Indyka, P.; Piskorz, W.; Kaczmarczyk, J.; Sojka, Z., Surface structure and morphology of M[CoM′]O4 (M=Mg, Zn, Fe, Co and M′=Ni, Al, Mn, Co) spinel nanocrystals-DFT+U and TEM screening investigations. J. Phys. Chem. C 2014, 118, 19085-19097. (40) Monkhorst, H. J.; Pack, J. D., Special points for Brillouin-zone integrations. Phys. Rev. B 1976, 13, 5188-5192.


Page 11 of 11 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC graphics


Valence Engineering via Selective Atomic Substitution on Tetrahedral

Recommend Documents